kinda out of the scope of this client-side framework.... but i suppose you could save the binary data into an abstraction that represents an image (file, perhaps?) and send it back to the browser with appropriate content type? i would google around to see what others have done for this approach as this is not specific to GXT framework.

If you want to have file uploading available in your Web site, whether intranet or Internet (although I prefer to use only in Intranet servers), you can build it all by yourself.
You can build your own ISAPI DLL; You can build your own COM/MTS DLL;
You can have your file upload transactional;
You don't need to have a directory with write permissions;
You can even deal with all your upload content only in ASP code (example appears in the article code).